All of my reasons have really been touched upon by others...but anyway, in no particular order:
1) I might lose my place to live. I live with my parents and they might kick me out. I'm saving my money to move out, but I have a few more months of saving to go before it is really possible. I think I would also feel much more comfortable if I were living on my own when I start to transition, I wouldn't have to feel like I'm hiding and I could really live how I want and I can't do that now.
2) I could easily get fired over it. I am committed to going back to school this fall to finish my degree, and my company will pay all my tuition and half of my other costs...so I really can't afford to lose that on top of losing my income...basically I know I'm costing my company extra cash, so even though I know I'm doing great there, I get paranoid that they'll look for reasons to get rid of me. I need more savings in the bank before I can take that risk.
3) It is going to cost a lot for me to start to physically transition. I am doing anything and everything I can right now that doesn't cost me anything, but that really isn't much

4) I'm absolutely terrified of how I will be perceived by society. I have no real self confidence as it is right now, so I kind of think I will collapse under not being seen as a person by some.
There are more reasons than these, little reasons, but these are the big ones. I'm really excited this year though because unless I screw up pretty big, I won't have to worry about 1 & 3 too much longer

One step at a time...
I know these are all things that are delaying me, not outright stopping me. I don't like having to deny myself my actual life, but I guess I feel like I need to secure my ability to ever be able to live that life first.